1. Field of the Invention
The present invention relates to a method and apparatus for electrophotography. More particularly, the present invention is directed to improvement in the method and apparatus for electrophotography in order to prevent developer particles from being scattered from the photosensitive medium into the surroundings.
2. Description of the Prior Art
Laser beam printers based on the electrophotographic process are known in the art and have the advantage that image processing can be performed at high speed. For this advantage, the 1aser beam printer has recently been employed widely as terminal printer of a computer system.
The optical system conventionally used in the electrophotographic copying machine is not necessary for the laser beam printer. The signal to be applied to it as terminal signal is signal-modulated laser beam. A high quality image can be obtained from the laser beam through substantially the same processings as required in the conventional copying machine.
In the laser beam printer, laser beam modulated by an information signal is applied to a photosensitive medium to form a corresponding image on it. For the laser application there has been generally used the so-called image scan method. According to it, the laser beam is applied on the photosensitive medium only at the portion corresponding to data signal such as character. To the remaining portion corresponding to background there is applied no laser beam. However, it is also known to apply the laser beam not to the image portion but to the background portion. This method is called background scan method. Compared with the background scan method, the first mentioned image scan method has the practical advantage that there is produced no trace of scan in the background image and there is no problem of runaway of data signal. For this advantage, the image scan system has been employed widely.
When the image scan method is applied to an image forming apparatus of the type using a Carlson type electrophotographic photosensitive medium, the formed latent image is developed by the reversal process. In the reversal development, there is used a developer charged with the same polarity as that of the latent image background portion of the photosensitive medium. The developer is applied to the latent image portion where the charge on the surface of the photosensitive medium has been decayed by the laser beam exposure. The developed image is transferred onto a transfer material for final use.
FIG. 1 schematically shows the basic arrangement of such laser beam printer according to the prior art.
Designated by 1 is a photosensitive drum comprising a photosensitive layer 2 and an electroconductive substrate 3 which is grounded. Around the photosensitive drum 1 there are arranged various processing stations only the main units of which are shown in FIG. 1.
4 is a primary charger. 5 indicates laser information light. 6 is a developing device provided with a doctor blade 7 and a magnet roller 8. 9 is developer. 10 is a pre-charger for transferring, 11 is a transfer sheet guide member and 12 is a transfer charger. 13 is a cleaning device comprising a rubber blade 14, magnet roller 15, screw 16 and casing 17.
The manner of operation of the apparatus is as follows:
At first the surface of the photosensitive drum 1 is uniformly charged by the primary charger 4 and then the drum surface is exposed to laser information light 5 the intensity of which corresponds to the terminal signal applied by the above described image scan. According to the applied information light, an electrostatic latent image is formed on the drum 1. After the exposure, the drum 1 enters the developing station where the latent image is developed by the developing device 6. At the next transfer station, the developed image is subjected to corona discharge by the pre-charger for transferring 10. Although not shown, a transfer paper sheet is supplied to the transfer station through the guide 11 in proper timing. Under the action of corona discharge from the transfer charger 12, the developed image is transferred onto the transfer sheet. After transferring, the residual developer on the drum 1 is removed off by the rubber blade 14 in the cleaning station. The removed developer is drawn to the magnetic roller 15 and then recovered in a toner recovery box by the screw 16. The toner recovery box (not shown) is formed as a part of the casing 17.
After cleaning, the drum 1 again enters the first step, that is, the primary charging step for the next cycle of operation.
FIG. 3 illustrates an example of the copy obtained by the above process. 18 is a copy having a developed image 19. The developed image 19 is composed of developer and is fixed on the copy 19 to form a permanent record.
As previously noted, in the above image forming process, the laser application step is carried out according to the image scan method. Therefore, the area of the developed image 19 on the copy 18 corresponds to the portion of the drum surface which was exposed to the laser information light. The background area of the copy other than the developed image 19 corresponds to the portion of the drum surface to which no laser application was carried out.
FIG. 2 is a curve showing the change of surface potential on the drum 1 with time at the latent image forming step in the apparatus described above. In the relation curve of FIG. 2, the surface potential is plotted on the ordinate and time on the abscissa. In the shown example, the photosensitive layer 2 of the drum 1 was formed of amorphous silicon and the photosensitive layer was primarily charged with negative polarity by the primary charger 4. As seen in FIG. 2, the applied surface potential by the primary charger 4 is changed to a latent image potential in the order of about 450 V as the contrast or the difference between the dark decay and the light decay caused by the laser exposure.
FIG. 4 illustrates the image after development. The surface potential on the drum is plotted on the ordinate. The abscissa indicates the direction of the length of the drum surface. The background V.sub.D is a negative potential portion having no toner adhered thereon. The laser exposed portion V.sub.L is at a potential near the ground potential and has negativecharged toner adhered thereon.
As previously noted, the latent image formed on the photosensitive drum is developed in the manner of reversal development. Consequently, the retention force with which the electrostatic latent image retains the developer thereon is relatively weak. In addition, in the shown type of apparatus, an electric field tending to separate the developer away from the photosensitive drum surface is produced between the drum surface and a member adjacent to it such as developing device, transfer guide or cleaner. Because of it, there occurs the phenomenon of scattering of developer in this type of apparatus. This undesirable phenomenon will be described in detail with reference to FIG. 5.
FIG. 5 schematically shows the imaginary lines of electric force produced between the drum surface 1 and a grounded conductor adjacent to the surface, for example, the casing 17 of the cleaning device. Since the polarity of charge on the drum surface is negative as a whole, the lines of electric force run toward the drum surface 1 from the tip end of the conductor 17. Therefore, an especially high density of electric force lines is produced at the tip portion of the conductor. The toner on the drum surface is, therefore, subjected to a drawing force which tends to draw the developer toward the open tip portion of the cleaner casing 17. Thereby, the developer particles are separated from the drum surface and the separated developer particles scatter over in the machine. The result is a contamination of the machine with particles.
To improve the retention of developed image while employing the above image scan system, there may be used the following electrophotographic method:
The method uses a photosensitive medium basically composed of an electroconductive layer, a photoconductive layer and an insulating layer. The surface of the photosensitive medium is primarily charged with a selected polarity and then exposed to the beam of light modulated according to the data signal in accordance with the image scan method in which said data portion means ON. Simultaneously with the beam exposure, a secondary charge with the opposite polarity to that of the primary charge is applied on the photosensitive medium by use of corona discharge means etc. with sufficiently high voltage being applied to said means. Thereafter, the whole surface of the photosensitive medium is subjected to a uniform exposure to form a latent image thereon.
In the electrophotographic method, the light portion (data portion) of the formed latent image has a high potential whose polarity is the same as that of the secondary charge. Therefore, by selecting a developer charged with the same polarity as that of the primary charge, the latent image can be developed without the above-mentioned trouble of scattering of developer.
However, this electrophotographic method involves another difficult problem. As a result of the secondary charge, there is produced at the dark portion an electric field with the opposite polarity. The top insulating layer and the underlying photoconductive layer of the photosensitive medium get in the state sandwiched in the produced electric field. This opposite polarity electric field leads to the problem that there remains some hysteresis in the layers of the photosensitive medium. When image formation is carried out continuously and repeatedly, it is made impossible to apply a sufficient amount of charge onto the portion having such hysteresis. In case of the normal development, the portion becomes white. In the reversal development described above, an unduly thickened portion results from such hysteresis. Such poor quality image is unacceptable. Because of this possibility of poor quality image, the electrophotographic method has hardly been used until now.
When the photosensitive medium of three layer structure was used, therefore, it was required to use a lower voltage for the secondary charge with the opposite polarity thereby reducing the possibility of hysteresis remaining in the photosensitive medium. Otherwise, it was required to carry out AC discharge instead of the application of voltage with the opposite polarity or to use AC biased by DC. However, even when these modified methods are employed, there occurs again the same problem of scattering of developer as in the above Carlson's system.
In any case, when the developed image on the photosensitive medium is weak in retention, there is often caused distortion of image at the transfer step. This distortion of transferred image is attributable to the fact that the transfer of the developed image takes place prematurely before the transfer material has sufficiently come close to or come into contact with the photosensitive medium. Thereby, the image is transferred on the transfer material in a position shifted from the position in which the image must be transferred when the transfer material and the photosensitive medium are in good state of contact for transferring. Some developer early separated from the photosensitive medium surface forms specks scattered around the transferred image. Thus, there is obtained distorted image on the transfer material.
Especially when the above Carlson's system is employed, the same polarity charge remains not only in the dark part but also in the light part. This is because the level of laser beam usually applied to the photosensitive medium for exposure is insufficient to completely erase the charge on the exposed part (light part). Since the polarity of such residual charge is the same as that of the developer applied to the light part, the developer is repelled by the residual charge. This enhances the problem of scattering of developer and therefore, the problem of distortion of the transferred image. The problem of residual charge occurs also even when the latent image is formed employing a three layer type photosensitive medium for the purpose described above. In this case, the residual charge is produced in the half-tone portion. The residual charge has the same polarity as that of the dark portion. Therefore, the residual charge repels the developer applied to the half-tone portion. In this manner, like the above Carlson process, this process also has the problem of scattering of developer and of the distorted image on the transfer material when reversal development is employed.